Subterranean sealing barriers are required for a variety of purposes and generally are intended to prevent migration of fluids from one side of the barrier to the opposite side thereof. While such barriers can be used as curtain walls, cofferdams and the like to prevent ground water from entering a protected area, they have found great importance recently as barriers to the passage of percolating, contaminating liquids from waste-disposal sites to protected regions around such sites.
In the past, it has been proposed to provide such barriers in the form of a so-called slit wall, i.e. a slit formed in the ground and filled with a barrier mass or provided internally with a barrier material which can be embedded in the mass. Generally speaking, such walls can extend down from the surface to rock strata or other geological formations which are impermeable to water and other fluids which may be contaminative or may be contaminating, to enclose the contaminated region and thereby prevent contamination of an adjoining area.
The sealing members in such systems may be vertically extending lamella or plates which are connected together contiguously, can have a thickness of 40 to 100 cm and a lamella width of, for example, 2 m or more.
The depth of the slit or structure can be 50 m or more, depending on the geological formations. The lamella can be composed of any nonpermeable material and the packing mass in which the sealing wall is embedded can be a mixture of bentonite, a hydraulic cement such as a Portland cement and water.
The purpose of the packing mass or mixture is primarily to support the slit and thus initially acts as a supporting fluid which then hardens to permanently anchor the sealing wall lamella in place.
As has been noted previously, in recent years, there has been an increased interest in such barrier structures to protect the environment against seepage from waste disposal sites. They are also valuable, of course, in conjunction with mining to prevent water from entering a particular region in which mining activity may be underway. For both of these purposes, the demand for a leakproof barrier has been significant and, therefore, the requirements for such barriers have increased significantly in recent years. In short, conventional or earlier barrier constructions have not been satisfactory because they lack the ability to provide the high degree of long-term guaranteed sealing effectiveness which is required.
In the past, sealing walls have been made from strips of polyethylene and sealing structures of this type have been termed multilayer systems. The individual strips of the sealing wall are formed into continuous barriers with seals between the units. Nevertheless the barrier walls thus created are unsatisfactory for many potential pollutants, for example, chlorinated hydrocarbons which have a significant capability of diffusing through barriers provided with polyethylene strips. Furthermore, sealing walls consisting of polyethylene webs constitute membranes in the physical sense and because of partial pressure on both sides of the membrane, it is not possible to guarantee that there will not be a material transfer across the membrane.